How can I cure my gravity circulation problem?

[Neville Hillyer]

I would provide this as proof of non performance to your supplier and ask them to show you a performing gravity fed coil of the same type somewhere near you.
It would be nice if a photo was available.

An interesting approach. I may end up doing this.

I discovered the slightly drooping coil just after delivery. As you indicated it is not easy to see through a small hole especially with light reflecting from internal surfaces. Even if a photo is possible it would probably have to be done with the cylinder removed from the airing cupboard - a long, but not impossible, task.
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I do not have a drawing of my system but will try and produce one later. Apart from changes to accommodate the new pipe connection locations everything is the same as it has been for the last 50 years.

I have attached the promised schematic. I do not know exactly what is between the ceiling (green) and the floor (brown). This 2D drawing is a reasonable guide to total pipe lengths especially between boiler and cylinder.

 

[Gasmk1]

Have you checked the cold feed coul be partially blocked stopping the Gravity

 

[Brambles]

Neville,

the person we use for advice on cylinder design for gravity flow Aga / Reyburn oil and solid fuel hot water systems is Dr Angelo Goonesekera at Sheffield University.

Looking over recent designs, the coil bore internal diameter is specified as a minimum of 28mm and the vertical differential between the inlet and the outlets of the coil is significantly larger than for conventional vented cylinders.

Perhaps worth comparing the dimensions of your original cylinder against your new cylinder.

 

[SJB060685]

Neville,

the person we use for advice on cylinder design for gravity flow Aga / Reyburn oil and solid fuel hot water systems is Dr Angelo Goonesekera at Sheffield University.

Looking over recent designs, the coil bore internal diameter is specified as a minimum of 28mm and the vertical differential between the inlet and the outlets of the coil is significantly larger than for conventional vented cylinders.

Perhaps worth comparing the dimensions of your original cylinder against your new cylinder.

You're one of a few I consider guru's. Its good to see even you seek help from others at times

 

[Neville Hillyer]

Have you checked the cold feed coul be partially blocked stopping the Gravity

I can flush all pipe sections.
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Neville,

the person we use for advice on cylinder design for gravity flow Aga / Reyburn oil and solid fuel hot water systems is Dr Angelo Goonesekera at Sheffield University.

Looking over recent designs, the coil bore internal diameter is specified as a minimum of 28mm and the vertical differential between the inlet and the outlets of the coil is significantly larger than for conventional vented cylinders.

Perhaps worth comparing the dimensions of your original cylinder against your new cylinder.

Thanks for the name - it could be very useful.

This is confusing. 1 inch (25 mm) often has an OD of 28 mm.

 

[John.g]

Neville,
I would give very serious thoughts to installing a circ pump (even just temporarily) on the cylinder coil where it returns into the boiler. You will then be more or less certain of proving that the fault is with the coil design, even if you just power it up while the boiler is firing to do so. If contemplating this, I would suggest a Wilo Yonos Pico 6M (or 5M, but a bit of a odd ball) simply because you can set the head on this down to as low as 0.1M (I will check my own later to see), It is not ultra expensive (~ £125) to prove your point.

On your present System: Does the boiler just fire up when hot water only is required and if so what stops the thermosyphon effect on the upper rads on the pumped circuit?, is there a anti syphon valve fitted?.

 

[Neville Hillyer]

Neville,
I would give very serious thoughts to installing a circ pump (even just temporarily) on the cylinder coil where it returns into the boiler. You will then be more or less certain of proving that the fault is with the coil design, even if you just power it up while the boiler is firing to do so. If contemplating this, I would suggest a Wilo Yonos Pico 6M (or 5M, but a bit of a odd ball) simply because you can set the head on this down to as low as 0.1M (I will check my own later to see), It is not ultra expensive (~ £125) to prove your point.

On your present System: Does the boiler just fire up when hot water only is required and if so what stops the thermosyphon effect on the upper rads on the pumped circuit?, is there a anti syphon valve fitted?.

Yes the boiler is called by the boiler thermostat when central heating is not called. No anti-syphon valve that I am aware of. I assume the stationary pump prevents thermosyphon to all radiators except bathroom.

 

[John.g]

A pump in clean condition will give virtually no resistance to flow, some zoned systems use pumps for zoning and all have anti syphon valves fitted which are spring loaded NR valves that require a slight pressure to open, unwanted thermosyphon circulation is often a indication of a seized valve so a bit strange that there is no thermosyhon effect in these rads as well as the bathroom one.
I presume that you have proven that there is no build up of sludge in the boiler by flushing through, is it gas or oil fired?. Oil fired boilers have virtually no resistance but the newer gas fired boiler Hxs do but I would think that the older type have some form of cast iron Hx which probably also has a very low resistance.

 

[Neville Hillyer]

A pump in clean condition will give virtually no resistance to flow, some zoned systems use pumps for zoning and all have anti syphon valves fitted which are spring loaded NR valves that require a slight pressure to open, unwanted thermosyphon circulation is often a indication of a seized valve so a bit strange that there is no thermosyhon effect in these rads as well as the bathroom one.
I presume that you have proven that there is no build up of sludge in the boiler by flushing through, is it gas or oil fired?. Oil fired boilers have virtually no resistance but the newer gas fired boiler Hxs do but I would think that the older type have some form of cast iron Hx which probably also has a very low resistance.

It is the original 50 year old pump so it may have a valve. Thermosyphon works in the bathroom radiator whenever the boiler is on which probably implies there are no blocked pipes. The gas boiler in a 100,000 Btu/hr cast iron Ideal Standard stopped down to 80,000 Btu/hr. At about 10 years old I had to clear cast iron sludge from the pipe under the first ground floor radiator but since then it has not been a significant problem. At about 10 year intervals, while replacing cylinders, I thoroughly flush and add new Fernox corrosion inhibitor.

 

[John.g]

Well, looks like a impasse if Telford claim that "their" coil is gravity suitable and you are reasonably/quite certain that it is not and you will have to pursue some option either legally or otherwise to get hot water.

 

[SimonG]

40 posts in now. Easier to pump it.

 

[Brambles]

This is confusing. 1 inch (25 mm) often has an OD of 28 mm.

The two designs I looked at, that were prepared for an indirect gravity hot water cylinder for solid fuel Rayburn ovens. They both had 28mm internal diameter coils specified extending over the lower two thirds of the cylinder.

 

[gmartine]

I only read the OP's first post and not much else, would the fact that the new cylinder is made from stainless steel rather than copper have a stalling effect being experienced because of the less efficient thermal transfer or are the coils made from the same material?

 

[Neville Hillyer]

I only read the OP's first post and not much else, would the fact that the new cylinder is made from stainless steel rather than copper have a stalling effect being experienced because of the less efficient thermal transfer or are the coils made from the same material?

The earlier copper cylinders were 100% copper and the new cylinder is 100% stainless steel. If you view conductivity on a log scale between rubber and metal, glass is somewhere in the middle and all metals are very close together at the end. Any difference between copper and steel conductivity is negligible in this application.

 

[John.g]

Above posts are interesting but while copper should give a faster warm up than stainless for the same coil area, using stainless should not cause stalling.

Is there any way that the coil has been installed somewhat like the attached even though the flow/return connections are shown at 600mm apart in your drawing.

Also, if you don't me asking, what do you intend to do to resolve your problem as you have lived with it for a year or so?.

 

[gmartine]

The earlier copper cylinders were 100% copper and the new cylinder is 100% stainless steel. If you view conductivity on a log scale between rubber and metal, glass is somewhere in the middle and all metals are very close together at the end. Any difference between copper and steel conductivity is negligible in this application.

If the rate of heat transfer is critical to how a thermosyphon works and yours appears particularly sensitive, irrespective of the scale compared to others if you then used a material that was subsequently 1/20 less efficient at doing so it isn't a negligible factor.

 

[Neville Hillyer]

Above posts are interesting but while copper should give a faster warm up than stainless for the same coil area, using stainless should not cause stalling.

Is there any way that the coil has been installed somewhat like the attached even though the flow/return connections are shown at 600mm apart in your drawing.

Also, if you don't me asking, what do you intend to do to resolve your problem as you have lived with it for a year or so?.

I saw a report of copper being faster but this was for identical metal thicknesses. Stainless cylinders are normally designed to withstand specified pressures which results in lighter and thinner cylinders and coils. In my case I am not concerned with speed. I have ample storage for my preferred relatively low primary temperatures.

As far as I could tell my coil approximates to a normal spiral between the connections. It is however supported by stainless wire and thin straps which, combined with the very flexible coil, caused it to 'sing' when moved empty just like moving a box of very light springs.

As a retired design engineer I am determined to do all I can to understand what is happening. Next I may try isolating the coil, disconnecting the primary from the boiler and power flushing each pipe in turn from roof to kitchen.

The attached schematic has been updated for: pipe colours and sizes, cylinder and pump isolation valves, non-return valve and pressure relief valve.
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If the rate of heat transfer is critical to how a thermosyphon works and yours appears particularly sensitive, irrespective of the scale compared to others if you then used a material that was subsequently 1/20 less efficient at doing so it isn't a negligible factor.

I am afraid I misunderstood. I thought your remarks were about energy efficiency rather than speed. You could have a point but I suspect this is not the cause of my problem.

 

[John.g]

Is the ~ 0.5M horizontal section of pipe hot right up to the coil inlet?.
You might also consider isolating the coil and install a 600 mm (spool) piece of pipe between the flow&return and see if you get thermosyphon circulation.

At the end of the day, if all else fails would you consider a pumped solution?.

 

[Brambles]

Neville,

I think that you have hit the nail on the head by looking at the calculations from first principles - I have never needed to calculate the pressure differential required to drive the thermosyphon effect before. It is a bit of an iterative process, but with water at 60 degrees C ( for the density) and a required ( my estimate) velocity of 0.5m/second the pressure differential to drive the flow 1meter is:

22mm inside diameter 1.00 inch head
28mm inside diameter 0.15 inch head

Thereafter for a 7m coil multiply the above differential by 7.

If you reduce the required velocity to 0.25m/second the equivalent pressure differential needed drops by a factor of 4.
At 0.125m/second the differential pressure required drops by a factor of 16.

There are rough estimates in the above for the friction factors and Reynolds Number. However, I was surprised by the significant influence that internal diameter has, and presumably in the longer term corrosion and silting of the flow pipework.

 

[SJB060685]

Neville,

I think that you have hit the nail on the head by looking at the calculations from first principles - I have never needed to calculate the pressure differential required to drive the thermosyphon effect before. It is a bit of an iterative process, but with water at 60 degrees C ( for the density) and a required ( my estimate) velocity of 0.5m/second the pressure differential to drive the flow 1meter is:

22mm inside diameter 1.00 inch head
28mm inside diameter 0.15 inch head

Thereafter for a 7m coil multiply the above differential by 7.

If you reduce the required velocity to 0.25m/second the equivalent pressure differential needed drops by a factor of 4.
At 0.125m/second the differential pressure required drops by a factor of 16.

There are rough estimates in the above for the friction factors and Reynolds Number. However, I was surprised by the significant influence that internal diameter has, and presumably in the longer term corrosion and silting of the flow pipework.

Brambles any chance you could provide a link to this calculation method?

 

[Brambles]

SJB

You don’t normally need it for general plumbing - the results are normally tabulated into charts such as the ones that give optimum flow rates for different diameter pipes.

If you want to calculate the values from scratch use the Darcey Weisbach equation:

P= f * v2 * p/2*L/D
P = pressure
f = friction factor
v2= velocity of the fluid squared
p = density of the fluid
L = pipe length
D= internal pipe diameter

f is determined (estimated) by calculating the Reynolds Number (Re)

Re = V * D *(p/vi)

vi is the viscosity of the fluid.
V is velocity of the fluid.

Published tables then give you the friction factor ( f )for a specific Reynolds Number (Re) to input into the first equation.

It is an iterative process so you need to do it a couple of times to be sure that the answer starts to be consistent.

I am unclear as to what the minimum velocity is for a gravity hot water system to be effective - but I would guess that the minimum flow rate that allows the boiler to operate properly is a key design criteria for the tank to match.

Does this make sense?

 

[SJB060685]

Yes makes perfect sense.
Like you said its not really needed to go into such detail but I'm the sort of person who doesn't like not knowing something exactly. If something like this pops up I will go and have a read through and mock up some rough figures just to get a feel for it

 

[Neville Hillyer]

Is the ~ 0.5M horizontal section of pipe hot right up to the coil inlet?.
You might also consider isolating the coil and install a 600 mm (spool) piece of pipe between the flow&return and see if you get thermosyphon circulation.

At the end of the day, if all else fails would you consider a pumped solution?.

The flow is about 40cm and the return is about 50cm to the tank connections.

Not as easy as it sounds but is is possible. However, I already have excellent thermosyphon circulation through the bathroom radiator. An easier trial would be to temporarily join the two pipes at the small header tank.

As indicated I would be very reluctant to abandon gravity circulation especially as it has worked well for 50 years.

 

[John.g]

I don't think joining the pipes at the header tank is a good idea as there will be virtually no static head at this point but as you stated there is excellent circulation through the rad and one might expect the flow pipe to be hot almost right up to the coil connection due to thermosyphon circulation within the pipe itself.

 

[SJB060685]

@Brambles can you say to me which units you were using during your rough calculations for the above required pressure.